Pulsed drive electromagnetic flowmeters rely upon Faraday's law, i.e. the voltage induced across a conductor as it moves at right angles through an electromagnetic field will be proportional to the velocity of that conductor. In the case of an electromagnetic flowmeter the conductor is the fluid and its velocity is inferred from the voltage induced across the flowing fluid.
With a pulse drive electromagnetic flowmeter the designer of the flowmeter has control over the energisation of the electromagnetic field. Usually the energising coils are driven at a constant current with alternating polarities, the frequency of which is under the control of the designer.
The signal levels generated by the flowmeter as a result of the flowing fluid are typically very low, for example ranging from nV to a few mV, and consequently interference rejection is very important. It is the rejection of periodic interference, such as mains-power related interference, which is the purpose of embodiments of this invention.
In practice, mains frequency interference may arise for example from pump or other machinery drives in the vicinity of the flowmeter, and thus is likely to be a problem in many industrial environments in which such flowmeters may be installed.
It is industry standard practice to ensure that any signal processing rejects interference from the mains frequency and its harmonics. However, from a product manufacturing perspective, this is complicated by mains frequencies differing from country to country around the world. In this respect, two mains frequencies predominate, namely 50 Hz and 60 Hz.
Current practice for signal processing is for the mains frequency to be set in the device, typically to either 50 Hz or 60 Hz. For instruments which are powered from a mains supply the mains frequency can be detected by the device itself, and consequently the interference (i.e. at the detected mains frequency and harmonics thereof) can be readily determined and rejected. However, for modern instruments, powered from say batteries or renewable energy sources, the device may have no way of determining the local mains frequency, making reliable detection and rejection of mains frequency interference difficult.
If a device is manufactured for sale worldwide, the intended mains frequency for the intended country of use can be set in the device at the point of manufacture, but if the device is ultimately used in another country in which the mains frequency is different (for example due to a shipping error by a third party reseller) then this can lead to interference problems and measurement inaccuracies. Alternatively, the mains frequency may by set the end user, e.g. at the point of installation, although the user may inadvertently fail to do this, again resulting in interference problems and measurement inaccuracies during use.
In the case of electromagnetic (EMF) flowmeters, interference rejection may be obtained by setting the coil excitation frequency to a sub-multiple of the mains frequency, such as one quarter of the mains frequency. For example, for 50 Hz mains frequency a coil drive of say 12.5 Hz (50 Hz/4) is typical, whereas for 60 Hz mains frequency it may be set to 15 Hz (60 Hz/4). However, as discussed above, if for any reason the instrument is operated with a local mains frequency other than that for which the device has been set, the rejection systems are unable to reject the mains related interference, and readings produced by the flowmeter can consequently be inaccurate and/or unstable.
There is therefore a desire to improve the design of such devices so as to prevent or at least mitigate such problems from occurring, for instance in the event that the mains frequency as set in the device does not correctly correspond to the local mains frequency.